The present invention relates to wheel alignment. In particular, the invention relates to an improved technique for calibrating sensors in a wheel alignment system.
Proper alignment of the wheels in an automotive vehicle is important for proper handling of the vehicle and proper tire wear. In addition, proper alignment of the wheels will decrease fuel consumption as well as increase vehicle safety. The wheel alignment parameters which are measured and adjusted in order to achieve proper wheel alignment are camber, caster, steering axis inclination and toe.
The static toe angle of a wheel, at a specified wheel load or relative position of the wheel center with respect to a sprung mass, is the angle between a longitudinal axis and a line of intersection of wheel plane and road surface. The wheel is "toed-in" if the forward portion of the wheel is turned toward a central longitudinal axis of the vehicle, and "toed-out" if turned away. Accurate toe adjustment cannot be attained without properly calibrated measuring instruments.
In measuring toe angle, measuring devices ("head units") are clamped onto the four wheels of the vehicle under test. On each side of the vehicle, the head units measure the front to back and back to front angles between each pair of wheels. The head units also measure the side-to-side angles between the front or back pair of left and right wheels. This angle is called the cross toe angle. U.S. Pat. No. 4,594,789 issued June 17, 1986 to Marino et al entitled "Wheel Alignment System" and U.S. Pat. No. 4,095,902 issued June 20, 1978 to Florer et al. entitled "Automobile Wheel Alignment Device and Method" are incorporated by reference. Both patents show a wheel alignment system using head units to measure cross toe angles.
In measuring cross toe angle, the head units must be properly calibrated to produce accurate measurements. Typically, this calibration is done at the factory during manufacture of the head unit. During operation of the wheel alignment system, however, the cross toe head units can change from the factory calibrated setting. This is caused by physical changes in the shape of the head units as well as drift in the electrical components. The operating environment of the typical service station where wheel alignment equipment is used is very stressful on precision measurement equipment. Head units are often bent, dropped, exposed to temperature extremes or otherwise stressed.
Should an operator of the wheel alignment system suspect that a head unit is out of calibration, a technician must be summoned to perform calibration tests. The technician must bring calibration equipment which is extremely precise. The process of recalibrating a head unit is involved and must be performed by a trained operator. Although cumbersome, the calibration equipment must be brought to the service station by the technician each time calibration is checked.
Recalibration also results in a significant down time for the wheel alignment equipment. If the operator suspects a calibration problem and summons a technician, the technician must schedule a time to bring the calibration equipment on site to recalibrate the alignment system. This can cause a delay during which the wheel alignment system is taken out of service and does not produce revenue for the service station. Ironically, the technician may find that the suspect head unit was not out of calibration in the first place.
Calibration equipment for cross toe head units in an automotive vehicle wheel alignment system which is inexpensive, easy to use and robust would be a significant contribution to the art.